What I have found so far:
- posteo looks good, but does not support custom domains
- tutanota can support it though I somewhat doubt their survivability long-term
- proton mail also supports it, but not a huge fan
- zoho: tried it years ago and it was ok, subprocesor list has aws and google on the periphery but not core
- ghandi.net: ?
- fastmail: ?
- kolab: ?
- Mailfence
Are there any others to look at?
I am not looking to self-host.
I’m sure they’re reading all my emails, but I use Yandex (the Russian equivalent of Google). Yandex is completely free even for multiple accounts (there’s a limit but I forget what it is). The interface works well and has an English option.